Add contributor instructions template

[hlapp]

Should create contributor guidelines document, and perhaps also a pull request template with some corresponding checkboxes.

This came up in the context of the following contribution:

@hlapp I ran make and make_release as instructed in the README-editors file and committed the two updated files in 0527d24. Since this repository doesn't have contributing guidelines, I wasn't sure if this was something I should do or not

Originally posted by @cthoyt in #5 (comment)

[cthoyt]

this exists: https://github.com/phenoscape/taxrank/blob/master/src/ontology/README-editors.md but it's a bit outdated. I guess we can also just change it to: "ping cthoyt or jim because they did this before and this doesn't need updating often" 😆